In a fuel cell, chemical reaction energy of a fuel is directly transformed into electric energy to generate an electric current. That is, the fuel cell includes an electrical power generating system in which the chemical reaction energy of the fuel (hydrogen or methanol) and an oxidation agent (oxygen or air) is directly transformed into the electrical energy.
The fuel cell continuously generates the electric current using the fuel supplied from the outside of the fuel cell without combustion or cycles of charge and discharge. The fuel cell has significantly higher energy efficiency than an electric generator using thermal dynamic energy produced by combustion of a fuel, because the fuel cell is not governed by thermodynamic efficiency limits.
Generally, the chemical reaction of a fuel (hydrogen) and an oxidation agent (oxygen) in a fuel cell produces water. Currently marketed fuel cells include polymer electrolyte membrane fuel cells (PEMFC) and phosphoric acid fuel cells (PAFC), which use an acidic electrolyte. Equations of the chemical reaction in a fuel cell in which an acidic electrolyte is used are as follows.cathode reaction: O2+4H++4e−→2H2Oanode reaction: H2→2H++2e−total reaction: 2H2+O2→2H2O
That is, when a fuel (commonly hydrogen) and an oxidation agent (commonly air) are respectively supplied to an anode and a cathode at the same time, energy is generated by an oxidation reaction of the fuel at the anode and an oxygen reduction reaction of the oxidation agent at the cathode caused by a catalyst, thereby resulting in the generation of electrons.
Efficiency of the catalyst is an important parameter to improve the efficiency of the fuel cell. Noble metals such as platinum, which is the most stable in an electro-chemical reaction, can be used as the catalyst. However, pure noble metals such as platinum may be too expensive to apply commercially in a fuel cell.
Therefore, several studies have been performed to develop an alloy-based catalyst that is capable of being substituted for pure noble metals such as platinum. For example, U.S. Pat. No. 4,447,506 discloses alloy catalysts such as Pt—Cr—Co and Pt—Cr and U.S. Pat. No. 4,822,699 discloses Pt—Ga and Pt—Cr.
The noble metals can also be supported (or coated) on a supporter such as carbon to reduce the amount used.
Japanese Patent Publication (No. 2002-42825) discloses a catalyst for a fuel cell having platinum particles of which 5 wt % or more are cubic shape and/or regular tetrahedron shape platinum particles to reduce the amount of platinum used and to improve efficiency. The reference discloses a method for preparing the catalyst that includes bubbling hydrogen in an aqueous solution formed by dissolving platonic chloride (II) potassium and sodium polyacrylate so as to generate a platinum colloid solution; adding a conductive carbon material to the platinum colloid solution; adjusting the pH level of the liquid mixture to be not greater than 3 or not less than 12; separating the colloidal particles from the liquid mixture along with the carbon material; and heat-treating the separated carbon material with the colloidal particles at 180 to 350° C.
Japanese Patent Publication (No. 2002-015745) discloses a catalyst in which a perfluorocarbon polymer including a sulfonic acid group and platinum or platinum alloy materials are supported in a carbon supporter. The fabricating method of the above-mentioned catalyst includes: dispersing carbon powders in an aqueous solution of platinum chloride acid; dissolving or dispersing a compound including elements which are capable of alloying with platinum when the platinum alloy is used instead of the platinum; performing adsorption of the compound including the platinum or the platinum alloy on the carbon powders through heating and stirring; controlling pH to the alkaline state if necessary; filtering, washing and drying the resultant; and performing heat-treatment under an inert gas atmosphere.
However, the catalytic activity of the above described platinum-based catalysts is not satisfactory and there remains a need for a catalyst for a fuel cell having an improved catalytic activity.